Final Report Abundance of Juvenile Lobsters at the New Outfall Site: Comparisons with Inshore Abundances and Discussion of Potential Outfall Impacts on Lobster Populations

Abundance of juvenile lobsters at the new outfall site: comparisons with inshore abundances and discussion of potential impacts on lobster populations. The Massachusetts Water Resources Authority (MWRA) was mandated by the Outfall Monitoring Task Force (OMTF) to design and execute a study in the cobble-boulder habitats of the new outfall nearfield region for determination of the density of early benthic phase lobsters (" EBPs " , 5 to 40 mm carapace length (CL)), particularly that of new recruits (" young-of-the-year " , <12 mm CL) and yearling lobsters (shelter-restricted, <20 mm CL). Both of these life history phases are thought to be relatively nonmobile, obligate shelter-dwellers. MWRA was also required to determine if the densities of these life history stages were comparable to that of nearby inshore habitats. This mandate resulted from serious concerns about the effects that the new outfall might have on juvenile lobsters and thus the future of the economically important lobster fishery. If densities of these relatively nonmobile life history phases were as high at the outfall as they were at inshore habitats, then it was recommended that MWRA consider further testing of effluent impacts on lobster juveniles. MWRA responded to this mandate by proposing a survey plan that incorporated previous data on lobster density from hard bottom surveys and used a mathematical calculation to determine appropriate sample sizes for the collection of species occurring only rarely in a region. Both of these tactics were designed to maximize the chances of locating young-of-the-year and shelter-restricted lobsters at the outfall vicinity. In early September 1998, density sampling was undertaken by the foremost experts in airlifting for lobsters underwater at both the vicinity of the outfall and two nearby inshore stations. The data collected showed significantly lower densities of young-of-the-year and yearling lobsters as well as larger EBP lobsters at the outfall compared to the inshore sites. Measures of the proportion of non-zero observations (which is another measure of frequency) for each size class also showed significantly fewer non-zero observations at the outfall. Taken together, these data demonstrate that while the cobble habitat at the vicinity of the outfall is suitable for settlement, it does not represent a major settlement site and thus there is no indication that the outfall will have any appreciable impact on these life stages of the American lobster.